The present invention generally relates to an automatic focus control system for use in a photographic camera and more particularly, to an automatic focus detection system in which two A/D converters which convert analog data representing defocus amounts into digital data are provided respectively for one bit and for a plurality of bits so as to be properly used, thereby to reduce processing time to be shorter than that in the conventional arrangement employing an A/D converter only for eight bits.
Conventionally, there has been proposed an automatic focus detecting arrangement including an image sensor which produces an analog signal corresponding to an image of a target object to be photographed, transmitted through a lens assembly, and an A/D converting means which converts the analog signal from said image sensor into a digital signal of eight bits so as to calculate a defocus amount from the eight bit digital signal for effecting the focus detection. In the above known arrangement, however, the defocus amount is calculated exclusively through employment of only the digital signal of eight bits, without effecting the calculation of the defocus amount by the digital signal of one bit (binary signal).
Meanwhile, there has also been conventionally proposed another automatic focus detecting arrangement which is adapted to effect the focus detection by the digital signal of one bit as disclosed for example, in Japanese Patent Laid-Open Application Tokkaisho No. 59-165012. In this known arrangemen,, the image signal in the analog form is converted into the digital signal of one bit for processing, and as a first step, the analog image signal is read by thinning-out. By way of example, in pixels of "a" pieces, data for CCD for each pixel is read out for A/D conversion so as to obtain a defocus amount through calculation, and thus, the lens assembly is driven by this defocus amount. After completion of the driving, the continuous analog image signals in 1/2 "a" pieces are read out for calculation to obtain a new defocus amount. The above prior art arrangement is intended to cover variation in size of the tagget object in a field to be photographed when a zoom lens is employed.
In the first prior art as referred to earlier, although accuracy for the focus detection is high when the defocus amount is small since signal processing for eight bits is effected at all times, there has been such a problem that it takes a long time before arrival at a final focus detection due to wasteful procedures involved, owing to the fact that the signal processing for eight bits is not necessarily required in the case where the defocus amount is large as in a first focus detection. Meanwhile, as in the photographing after the first exposure in a continuous photographing mode, the signal processing for eight bits is effected at all times even when such signal processing for eight bits is not necessary, thus requiring much time for the focus detection, with a consequent limitation to the speed in the continuous photographing. Moreover, as described later, there are also many cases where the signal processing for eight bits is not required or where such eight bit signal processing is not preferable, and in the prior art technique as referred to above, since the eight bit signal processing is effected for all of these cases, the focus detecting speed is undesirably limited.
On the other hand, in the second prior art as referred to earlier, although signal processing for one bit is disclosed, this is not a technique related to the high speed focus detection, and thus, the problems in the above first prior art can not be solved thereby.
In order to overcome the disadvantages in the prior art techniques as described above, the present inventors have thought of an arrangement in which, in a prior art automatic focus detection system, a data processing means for one bit is provided apart from the data processing means for eight bits in order to increase the processing speed for the focus detection, thereby to properly utilize these two data processing means through change-over therebetween. In this case, when the data processing for one bit is selected, the calculating speed is of course increased due to the small number of bits, but it has been further studied by the present inventors as to whether or not there are any othe points in which one bit and eight bits can be exchanged, thereby to increase tthe processing speed for the overall focus detection. As a result, it has been found that the processing speed in one bit may be increased with respect to the following two points.
(a) Since the data only for one bit is required, the information (output voltage) of the CCD image sensor may be smaller than that in the case where data in eight bits is employed. In other words, the output voltage thereof may be lower. This means that the integration time for obtaining the CCD information may be short. PA1 (b) Time required for the A/D conversion may be short as compared with that in the case of eight bits. Accordingly, it becomes possible to accelerate the period for the data taking-in, with a consequent reduction of time required for the data reading-in.